1. Field of the Invention
The present invention pertains to electronic packaging technology, and more particularly, to the packaging of modules comprising silicon chips mounted on a substrate disposed under a cap.
2. Description of the Prior Art
Electronic packaging technology generally includes the packaging of silicon chips within a module, the module including a substrate, the silicon chips mounted on the substrate, and a cap disposed over and enclosing the substrate inclusive of the chips. One such electronics packaging technique is disclosed in U.S. Pat. No. 4,312,116 to Moser et al assigned to the same assignee as that of the present invention.
The specification of U.S. Pat. No. 4,312,116 is incorporated by reference into the specification of this application.
In the Moser et al patent, silicon chips are mounted on a substrate, and a cap is disposed over the substrate, enclosing the substrate inclusive of the chips. However, subsequent to the development of the invention disclosed in the Moser et al patent, the density of the circuits on the chips has increased as a result of a demand for increased performance in modern day computing systems. Since a certain amount of power is dissipated within a chip, the increased circuit density on the chips has resulted in an increase in the heat dissipated by the chips. In order to prevent damage to the chips due to excessive heat dissipation, a heat dissipating means, such as a thermal grease, is disposed in the space between the chips and the top of the cap. The thermal grease carries away the heat dissipated by the chip.
As taught by the Moser et al patent, a gap exists between the edge of the substrate and the inner surface of the cap. An epoxy sealant is disposed over the back side of the substrate and is allowed to flow over the entire back side of the substrate and into the gap between the edge of the substrate and the inner surface of the cap. The epoxy within the gap adheres to the edge of the substrate and to the inner surface of the cap. As a result, the epoxy seals the module thereby protecting the chips disposed within the module from harmful external influences.
However, when the module is exposed to variations in external temperature, both the cap and the substrate tend to expand and contract, but at different rates. The epoxy within the gap prevents the cap from expanding and contracting independent of the substrate. As a result, the cap distorts. Since the cap is in contact with the chip via the heat dissipating means (the thermal grease), the distortion of the cap places an unacceptable degree of pressure or stress on the silicon chips mounted on the substrate. The chips are connected to conductor strips or lands on the substrate via a plurality of solder balls. The pressure placed on the chips tends to produce cracks within the solder balls. As a result of these cracks, an open circuit may be produced between the silicon chip in question and the conductor strip to which it is connected.